Response Characteristics of Coal Mechanical Behavior to Microwave Irradiation: A Uniaxial Compression Experimental Investigation

The application of microwave irradiation in low permeability coal seams is considered to be an effective method for increasing the permeability of coalbed methane (CBM). Microwave irradiation not only improves the pore structure of coal, but also affects the mechanical properties. To evaluate the impact of microwave irradiation on the mechanical behavior of coal, a series of uniaxial compression tests of coal after microwave irradiation with different parameters were carried out. The results suggested that the mechanical properties of coal were significantly reduced by microwave irradiation with different parameters. Microwave irradiation changed the deformation behavior and fracture properties of coal. After microwave irradiation, the stress–strain curve of coal tended to be flat, and the failure mode changed from shear failure to tensile failure. Peak strength, initial modulus, and elastic modulus were negatively correlated with microwave energy, while peak strain was positively correlated. The energy evolution of coal under uniaxial compression was closely related to the deformation behavior. The energy absorption, storage and dissipation capacity of coal showed a downward trend with the increase in input microwave energy. Moreover, the energy distribution ratio ( U_p^e/U_p^d ) of the stress peak point was changed after irradiation. Under compression, the deformation and failure characteristics of coal showed significant enhancement of plasticity and ductility. The brittleness index ( B 1 , B 2 ) first increased and then decreased with the increase of microwave energy, and the microwave energy of 90 kJ was taken as the threshold boundary. It was observed that the mechanical behavior of coal after microwave irradiation is related to the degree of microwave thermal damage. The average decay rate of normalized mechanical parameters with microwave energy is 0.0026 kJ −1 .